How do I repair a cracked concrete drive?

Telling a shrinkage crack from a structural one, cutting out and filling properly, and when a drive is past repairing.

Difficulty: intermediate
Time: 4 hr
Read: 3 min
Safety: caution

Short answer

Fine hairline cracks are shrinkage and need only sealing. A crack wider than about 3mm, or with a level difference across it, is structural: cut it out to a square profile, prime it, and fill with a polymer-modified repair mortar. Widespread cracking with movement means the slab has failed.

Concrete cracks. Most of it is shrinkage as it cured and it is cosmetic. The cracks that matter are the ones that are still moving — the ones that get wider, that show a step across them, or that follow the line of something buried underneath.

Safety

Cutting or grinding concrete releases silica dust — cut wet or with extraction and wear an FFP3 mask, eye and ear protection. Cement and concrete repair mortars are caustic; wear waterproof gloves and rinse skin immediately. Do not attempt to repair a drive that has dropped or opened over a drain run or a mine shaft area — get it investigated.

Step by step

Classify the crack.Hairline and stable, following no pattern: shrinkage, cosmetic. Wider than 3mm, or with one side lower than the other: structural. Cracks radiating from a corner or along a buried drain: something is moving underneath.
Monitor before repairing anything significant.Mark the crack ends with a pencil and date them, or fit a simple crack monitor. Two months of no movement means it is safe to fill. Continuing movement means the cause has to be found first.
Look for the cause of a moving crack.A leaking drain washing out the sub-base, tree roots, a collapsed trench backfill from a service run, or subsidence. Filling the crack without dealing with the cause is wasted work.
Cut out a repairable crack to a square profile.Widen the crack with a disc cutter or a grinder to at least 10mm wide and 10-15mm deep, with parallel or slightly undercut sides. A tapered V-shape lets the filler pop out.
Clean and prime.Vacuum out all dust — repair mortar will not bond to dust. Prime with the manufacturer's bonding agent or an SBR slurry, and fill while the primer is still tacky.
Fill with a polymer-modified repair mortar.Ordinary sand and cement shrinks and cracks straight back out. A polymer-modified or resin-based repair mortar bonds properly and tolerates a little movement.
Finish flush and texture to match.Strike off level with the surrounding slab and brush or float the surface to match the existing texture before it goes off.
Seal an expansion joint properly.Where the crack has formed at an intended joint, or you are cutting a new one, fill with a flexible polyurethane joint sealant rather than a rigid mortar. Joints are meant to move.
Consider an overlay if the whole slab is crazed.Widespread fine cracking on a sound slab can be covered with a resin-bound or micro-concrete overlay. A slab with movement and level differences needs breaking out and relaying.

Tips

Repair mortars have a temperature window. Below about 5C most will not cure properly — leave the job for milder weather.
Colour matching concrete repairs is close to impossible. A repair that is neat and flush looks better than one that has been tinted badly.
If the drive was laid without expansion joints, cutting joints in at 3-4m intervals reduces further random cracking.

Common mistakes

Filling a crack with sand and cement — It shrinks as it cures and does not bond well to old concrete, so it cracks along the same line within a year. Use a polymer-modified repair mortar.
Filling a moving crack — Whatever caused the movement is still there, so the filler cracks again. Monitor first, find the cause, and fix that.

If it doesn't work

Repair cracked again along the same line

Cause: Ongoing movement, or a rigid filler in a joint that needs to move — Fix: If it is a movement joint, use a flexible sealant. If the slab is genuinely moving, investigate the sub-base and drainage.

Filler lifting out in pieces

Cause: Dust left in the crack or no primer — Fix: Cut out, vacuum thoroughly, prime, and refill. Bond failure is nearly always a preparation failure.

Crack widening season by season

Cause: Sub-base washout or ground movement — Fix: Investigate drainage and check for leaking drains or downpipes discharging under the slab. This is not a filling job.

Surface crazing across the whole drive

Cause: Concrete finished too wet or dried too fast when laid — Fix: Cosmetic. A sealer or a bonded overlay improves the look; the slab itself is usually sound.

Questions people ask

When is a concrete drive beyond repair?

When there are level differences across cracks, when sections rock under a car, or when cracking is widespread and still moving. At that point the sub-base has failed and patching the surface achieves nothing.

Can I resurface a cracked concrete drive?

A micro-concrete or resin-bound overlay works over a sound slab with only cosmetic cracking. It will not bridge a moving crack — that will telegraph straight through the new surface.